1. Field of the Invention
This invention relates to an electrophotographic photoreceptor. More particularly, it relates to a photoreceptor applicable to LED printers, digital copiers and digital image output apparatus such as laser printers.
2. Description of Related Art
The Carlson-type electrophotography utilized in printers or copiers (hard copy imaging systems) is based on a combination of a photoreceptor having photoconductivity and a technique of electrostatic charging. A surface of the photoconductive photoreceptor is evenly charged in the dark by means of a corona charging or similar device and is then irradiated with optical information, whereby the static charge is removed from the parts corresponding to the light image to form an electrostatic latent image corresponding to the light image. The latent image is developed with a colored charged powder to visualize the latent image.
The basic properties of a photoreceptor for use in electrophotography include the ability to be charged in the dark to an appropriate potential, the ability to retain the charged state for a given period, and the ability to enable the charges to be rapidly neutralized upon light irradiation.
Inorganic compounds such as amorphous silicon, amorphous selenium, cadmium sulfide, and zinc oxide have conventionally been used widely as preferred photoconductive materials for such electrophotographic photoreceptors.
None of these materials has proven to be adequate. Amorphous selenium and cadmium sulfide are harmful to the human body, have been designated as pollutants and have been prohibited from use in the future. Amorphous silicon has a high production cost because it is produced by a special vapor deposition technique.
Recently, investigations on organic photoreceptors as substitutes for inorganic photoreceptors have been made. Various compounds for use as organic photoreceptors have been proposed. Among these are phthalocyanine pigments, which have come to be used in photoreceptors for laser printers and related devices because of their sensitivity to light having long wavelengths.
However, the organic photoreceptors currently used in the market have various drawbacks. They are fabricated as two-layer photoreceptors composed of a charge-generating layer and a charge-transporting layer. Since these multilayered photoreceptors are negatively charged in use, the corona charging device for negative electrification generates about ten times the ozone than that generated by a corona charging device for positive electrification. The excess ozone must be removed to meet the requirements of the Industrial Safety and Health Law. In addition, ozone attacks the photoreceptor surface to, causing a decrease in photoreceptor life.
In contrast to photoreceptors of the negative electrification type, single-layer photoreceptors are usable in a positively charged state. Because of this, many proposals have been made for the improvement of a photoreceptor containing a phthalocyanine pigment.
For the purpose of improving the printing durability of a photoreceptor, a mixture of an acrylic resin and a melamine resin was used in Japanese Patent Laid-Open No. 15250/1984 and 219752/1984. In Japanese Patent Laid-Open No. 207145/1985, a mixture of a polyester resin, a polycarbonate resin, and an acrylic resin was used as a binder in order to improve moisture resistance.
As a means for improving sensitivity, use of a mixture of poly-N-vinylcarbazole and a polyester resin as a binder was disclosed in Japanese Patent Laid-Open No. 185044/1982.
In Japanese Patent Laid-Open No.105550/1984, a specific phenolic resin was used for sensitivity improvement. Another technique for sensitivity improvement incorporates an electron-accepting substance into a photosensitive layer, as disclosed in Kitamura and Komon, "Denshi Shashin Gakkai-shi (Journal of The Soc. of Electrophotography)," 20 (2) 10 (1982) and Kitamura and Komon, "Denshi Shashin Gakkai-shi," 20 (3) 2 (1982). In Japanese Patent Laid-Open No. 187248/1988, a polyester resin and a melamine resin were used as a binder together with a modified phthalocyanine in order to improve durability.
The functions required of a photoreceptor for printers include high durability and high sensitivity. This durability is expressed in terms of the total number of prints that the photoreceptor can yield while retaining its printing function and an acceptable image quality. A photoreceptor is chemically deteriorated during image printing by the ozone generated by corona, and it suffers surface wear due to mechanical friction during development, cleaning and paper transfer. This causes scratch marks, resulting in reduced image quality. In addition, the thickness of the photosensitive layer decreases, causing a decrease in electrification potential and an increase in fogging. None of the conventional multilayered photoreceptors has a durability of 100,000 sheets or higher under these conditions.
On the other hand, wear resistance in conventional single-layer photoreceptors has been obtained by binder improvement. However, their sensitivity has still been insufficient in practical use. Although an essential requirement for a high-speed printer is that the photoreceptor combines high sensitivity with high durability, the attainment of the two properties with any conventional technique for photoreceptor production has not been achieved.
Prior art organic photoreceptors have been regarded as unsuitable for high-speed printers because of their insufficient durability. The inventor has investigated improvements of single-layer photoreceptors for many years. Attempts were made to improve the durability of a photoreceptor by heightening its surface hardness. As a result, the inventor succeeded in improving the surface hardness of a photoreceptor to 3H pencil hardness. With respect to durability, it was found that the photoreceptor surface had not developed scratches leading to a decrease in image quality even after a 300,000-sheet printing,
However, a high-speed printer is required to exhibit not only durability, but also high sensitivity and attenuation of surface potential upon exposure to a small amount of light. A conventional technique for imparting high sensitivity to a single-layer photoreceptor has been to increase the proportion of its photoconductive pigment. For example, in a single-layer photoreceptor containing a phthalocyanine pigment, increasing the proportion of the phthalocyanine pigment improves photosensitivity but accelerates dark decay to the point that the surface becomes potentially unstable. In addition, the pigment dispersion has abnormal viscosity characteristics, causing coating troubles.
Therefore, with respect to achieving high sensitivity in a single-layer photoreceptor, it is necessary to find a method for maintaining sensitivity even when a phthalocyanine pigment is incorporated in a small proportion to obtain a high-sensitivity photoreceptor with a desirable balance of all properties. The inventor felt a need to develop a new synthetic resin for use as a binder for a phthalocyanine pigment, and investigated this subject.